The use of optical waveguides formed by semiconductor manufacturing techniques has exploded in recent years. Both passive optical waveguides and active optical waveguides have found extensive application in communications and other fields. A passive optical waveguide is an optical waveguide that carries optical signals without providing amplification of the optical signal. In contrast, an active optical waveguide is an optical waveguide that provides some measure of amplification of the optical signal carried within the active optical waveguide.
Typically, a passive optical waveguide can be formed from a glass-like core material, such as silicon dioxide, surrounded by a cladding material. Similarly, an active optical waveguide has a core material that is surrounded by a cladding material. The core material is typically doped with a rare earth element, such as erbium. Thus, the core material is typically silicon dioxide doped with erbium.
In both passive and active optical waveguides, the intensity of the optical signal is attenuated by various effects, such as scattering loss associated with a surface roughness in the interface between the core material and the cladding. The attenuation caused by surface roughness is significant and is undesirable.
Prior art optical waveguides are exemplified by U.S. Pat. No. 5,900,057, U.S. Pat. No. 5,206,925, U.S. Pat. No. 5,693,116, and U.S. Pat. No. 5,119,460. In each of these prior art optical waveguides, the core material which carries the optical signal has substantially a rectangular cross sectional shape. The core material is of rectangular shape for apparently no other reason than the rectangular shape inherently results from the semiconductor processes used to manufacture the optical waveguide, such as deposition and etching.
Specifically, in one prior art process, the core material is deposited onto a lower cladding material and dry etched to form a “ridge” structure. Next, an upper cladding layer is deposited over the core material so as to surround the core material. In order to have an effective optical waveguide, the upper and lower cladding layers must have an index of refraction that is lower than the core material. This technique is exemplified by U.S. Pat. No. 5,119,460.
Alternatively, a damascene process may be used to form the prior art optical waveguides. Specifically, a lower cladding material may have trenches etched therein to carry the core material. The core material is deposited into the trenches and then, using chemical mechanical polishing, the core material outside of the trenches is removed. The upper cladding material is then deposited thereby surrounding the core material. In both of these techniques, because of the deposition and etching techniques used, the optical waveguide has a rectangular shaped core.
In each of these processes, an etching process, such as reactive ion etching (RIE), is used. The use of RIE on oxide typically produces sidewall roughness on the order of 20-50 um surface roughness. However, good optical performance requires 5 um (or better) surface roughness to avoid scattering loss during light propagation. Thus, the prior art methods produce a less than optimum optical waveguide.